/Engine/Source/Runtime/Engine/Classes/Materials/Material.h
A parameter with duplicates has to update its peers so that they all have the same value. If this step isn't performed then the expression nodes will not accurately display the final compiled material.
virtual void PropagateExpressionParameterChanges ( const [FName](API\Runtime\Core\UObject\FName) & ParameterName, const [FMaterialParameterMetadata](API\Runtime\Engine\FMaterialParameterMetadata) & Meta )